There are various techniques for removing dissolved substances (e.g., salts) from their solvents (e.g., water), but in recent years, membrane separation processes have been actively used in water treatment fields as low-cost processes for saving energy and resources. Typical membranes for use in membrane separation processes are microfiltration membranes, ultrafiltration membranes, nanofiltration membranes (NF membranes), and reverse osmosis membranes (RO membranes).
Most of RO membranes and NF membranes are composite semipermeable membranes, and most of them are divided into two types: one has a structure in which a gel layer and a thin film layer (separation functional layer) formed by cross-linking a polymer are provided on a microporous support membrane; and the other has a structure in which a thin film layer (separation functional layer) formed by polycondensation of a monomer is provided on a microporous support membrane. As materials of these thin film layers, cross-linked polyamides are often used. Among them, a composite semipermeable membrane, such as one disclosed in Patent Document 1 or 2, produced by coating a microporous support membrane with a thin film layer made of a cross-linked polyamide obtained by the polycondensation reaction between a multifunctional amine and a multifunctional acid halide is widely used as a reverse osmosis membrane or an NF membrane due to its high water flux and high salt rejection.
In addition to salt rejection performance, ion selective separation performance can also be considered as a factor having an economic impact on water treatment using RO and NF membranes. For example, in a case where a membrane has low selective separation performance even though the membrane is required to allow the passage of monovalent ions but not of divalent ions, the concentration of ions is excessively increased on one side of the membrane and the osmotic pressure on that side of the membrane is increased. When the osmotic pressure on one side of the membrane is increased, ions increasingly try to pass through the membrane to balance the pressure on each side of the membrane, and therefore a higher pressure is required to allow desalinated water to forcibly pass through the membrane. This consequently requires a large amount of energy and therefore increases costs for water treatment.
Currently-used RO and/or NF membranes do not satisfactorily achieve selective separation of divalent ions over monovalent ions, and therefore overall salt rejection rate is high and a large osmotic pressure difference is created across the membrane. Accordingly, a higher pressure, that is, a larger amount of energy is required to achieve a practical flux. For this reason, currently-used RO and/or NF membranes are not satisfactory from the viewpoint of energy conservation.
On the other hand, in the fields of materials, organic-inorganic hybrid materials are known, which are obtained by combining a hydrophilic organic polymer and a condensation product of a silicon compound by utilizing molecular interaction (see, for example, Patent Documents 3 and 4 and Non-Patent Document 1). However, such materials have not been previously used in industrial applications.